Method for producing cathode coating compositions



Patented Dec. 26, 1950 METHOD FOR TRODUCNG. CATHODE C OATING "-COMPO SITIONS George H. .Bouchard Ipswich, Mass.; assignor tto Sylvania. Electric Products .Inca Salem, Mass.,

a corporation of Massachusetts "'NolDrawing. IApplicationMay I2, 1945, ;fSerialNo. 593,535

s claims. (o1. "10c- 286)` Y -Ths invention relates to cathode coatingsior `electron tubes, lamps.. and the like and. particu.- larly to carbonatos with Whichsuch .cathodes are generally coatedprfior to activation.

I have found atriple carbonate, or mixture f barium, strontium andfcalcium `carbonatos to be quite effective,`although the inventionicanbe used A`with othersuitable-carbonatos also. The carbonatos may be formed by precipitation from an ammoniacal:'solution ofthe nitrates with arnmonium carbonate. The partcle size and purity ofthe resultant precipitate are quiteirnportant, the purity desired being as high as possible and the particle size as small as possible.

It is..accordingly an objectief this inventionto produce a carbonate of high purity and small particle size. A further object is to achieve this Without the need for criticalvalues of timestamperature, agitation, and rate of. .additionioi lammonium carbonate. In fact, I found that even if these variables vwere all kept constant, the particlesizewouldstillbe variable. l I have found `that if Alarge Aamount :of -ammonia was added just before precipitation, LI could `obtain `small particle :sizefand high purity, even despite considerable variationinithe other- Wise critical factors already mentioned.

A feature of the invention is accordingly the addition of ammonia just before precipitation.

Some ammonia may be lost by volatilization but the results are quite remarkable. The particle size may be controlled by the amount of ammonia used. For example, four batches made With a small amount of ammonia, keeping the critical factors xed, varied from 1.17 to 1.70 in bulk density reciprocal (cubic centimeters per gram) While four batches with my large excess of ammonia varied only from 2.69 to 2.86. The particle size was thus not only much smaller with a large amount of ammonia but more uniform and reproducible.

I have found that without ammonia, the particles are large and chubby, about 14 microns in dimension, and if the concentration of ammonia is less than 0.15 molal the particles are still large, of about to 9 microns size. If the concentration is 0.3 or more, the particles are largely needlelike crystals and quite small. An excess of 0.6 produces very small particle size. Some of the needle-like crystals will be long, thin needles, and in some the needles Will join to form V- shaped and Y-shaped particles, and some with a few additional arms than the V or Y have. But all have the same needle-like structure which clearly differentiates them from the larger,`

only a little,;is present. Afsingle needle ,willibe about limicron :wide: and 4.5 i microns long. In practising r the invention `I :may l dissolve, for example,.16 poundsiofbarium nitrate, 11 pounds 2 `ounces1of strontium nitrate, `:and 4 `pounds 1 ounceof -`calcium nitrate inAO :gallons ofywater at room temperature. A `motor stirrer :may .be used ifldesired to insure that `the .nitratesfare thoroughly-dissolved. `250 cc.of nitric Aacid'is then added, :and hydrogen sulphide is passed through .thesolution at the rate cf.2 or3 bubbles pertsecondufor 30 lminutes, The `solution A.may then be covered,and'allowedtofstand for 2 hours, after which the `solution is lteredto removelthe precipitate which may be discarded.

r'.li1e.itrate is then heated to 95" C. and 1400 cc. Vof ammonium hydroxide is added. 250cc. of, ammoniumsulphide is. then `added vand stirred. The heat maythenbe turnedpff 'and thesolution -allowedyto stand over night. Thesolution may athen `be riiltered, .for example, `through an Ertellterpress, and should be .cooleddown .to room temperature before filtering. -450 `cc. LAof hydrogen peroxide is then added;` andthe solution stirred. After Sstanding `over .night the solution is.; again filtered Vfor example, :through .an .E rtcl lter press.

The ammonium carbonate mixture which will eventually be added to the foregoing iiltrate for precipitation may be prepared by adding 18 pounds 12 ounces of ammonium carbonate, 500 cc. of ammonium hydroxide, and 100 cc. of hydrogen peroxide to 10 gallons of distilled Water. This solution when at room temperature is filtered through Buchner funnels. The solution is then covered and heated slowly to about C. The heating process may, for example, take about 2 hours.

Meanwhile, the puried nitrate solution previously described may be heated to C. This should take about 60 minutes to heat and be timed to reach temperature at the same time as the ammonium carbonate. The nitrate solution should then be stirred and 3 liters of ammonium hydroxide added to it. The previously prepared carbonate solution should then be added to the nitrate solution, for example, by being dipped into that solution through a Buchner funnel. The ammonium carbonate should be added immediately after the ammonium hydroxide in order to prevent loss of the hydroxide. The ammonium carbonate should be put into the solution as soon as possible, preferably in a continu# ous stream at a time of about 5 minutes., The

solution should be stirred while the carbonate is being added. The stirring may be stopped when the precipitation is complete and the precipitate: allowed to settle, for example, about 15 minutes: or until the top liquor is clear.

The clear top liquor may then be pumped out and the precipitate rinsed thoroughly with distilled Water. The precipitate is then ltered and dried. In preparing the nitrate solution the hy-v drogen sulphide and. the ammonium sulphide removes undesirable metal impurities. The hydrogen peroxide then removes any residual sulphides.

In the ammonium carbonate solution the hydrogen peroxide is present to oxide any ferrous impurities present to the ferric state so that they will be insoluble and precipitate out. The ammonium hydroxide in the carbonate solution prevents eiervescence when the carbonate is heated.

The use of ammonium hydroxide in the nitrate solution has the advantage in addition to its eiect on particle size, that it eliminates the effervescence which would otherwise occur when the ammonium carbonate is added. This allows doubling the capacity of the manufacturing equipment because room does not have to be provided for the effervescing gases, whose foam otherwise tends to carry the precipitated powder lout of the tank in which it is being precipitated.

My process produces a triple carbonate subf stantially free from sodium and potassium, which do not appear present even on spectrographic analysis. This is quite important if the carbonate is used to coat ultra-high frequency tubes and pulseproducing tubes. plied to the cathodes in the usual manner with lacquers, or with the centrifuge method described in a patent application Serial No. 595,309 led May 23, 1945 by William P. Toorks for Cathode and Method of Making, now Patent No. 2,433,821.

With ordinary methods, C. P. chemicals are used and puried, but my purication method allows the use of ordinary technical grade chemicals at considerable reduction in cost.

What I claim is:

1. The method of controlling the particle size and shape of triple carbonate crystals of the alka- The coating may be apline earth metals precipitated from a nitrate solution by the addition of ammonium carbonate which comprises adding a large amount of ammonia to the nitrate solution just before precipitation to produce the desired particle size.

2. The method of producing a powder of barium, strontium and calcium carbonates which comprises the steps of dissolving the nitrates of these metals in water, adding a large amount of ammonium hydroxide to the nitrate solution just before precipitation and then adding a solution of ammonia and ammonium carbonate to cause precipitation.

3. The method of producing a cathode-coating powder of barium, strontium and calcium carbonates which comprises the steps of dissolving the nitrates of said metals in water removing undesirable metallic impurities by the addition of hydrogen sulphide and ammonium sulphide to precipitate them out as impurities and ltering the same, adding hydrogen peroxide to remove the residual sulphides and filtering the same, heating the resultant nitrate solution, adding a large amount of ammonium hydroxide to the resultant nitrate solution and then precipitating the metallic carbonates by the addition of ammonium carbonate.

GEORGE H. BOUCHARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 301,383 Mebus et al July l, 1884 805.581 Pennock Nov. 28, 1905 1,865,437 Fredenburgh July 5, 1932 1,939,075 McCulloch Dec. l2, 1933 OTHER REFERENCES A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Mellor, vol. I, 1922, page 941.

Analytical Chemistry, TreadWell-I-Iall, vol. I, 7th ed., 1930, page 120` 

1. THE METHOD OF CONTROLLING THE PARTICLE SIZE AND SHAPE OF TRIPE CARBONATE CRYSTALS OF THE ALKALINE EARTH METALS PRECIPITATED FROM A NITRATE SOLUTION BY THE ADDITION OF AMMONIUM CARBONATE WHICH COMPRISES ADDING A LARGE AMOUNT OF AMMONIA TO THE NITRATE SOLUTION JUST BEFORE PRECIPITATION TO PRODUCE THE DESIRED PARTICLE SIZE. 